Adding a resistor wont help much, because battery has a variable inner-impedance (it rises while charging) so the current wouldnt stay constant. You can use LM317 as a constant curent regulator (for charging from NiCd or wall adapter).
I is really easy, all you need is lm317 and one resistor
http://linear1.org/gm/archives/00000147.php
And you can add a charging led parallel to the resistor....